Spotting green tides over brittany from space: three decades of monitoring with landsat imagery

Louise Schreyers*, Tim van Emmerik, Lauren Biermann, Yves François Le Lay

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

8 Citations (Scopus)

Abstract

Green tides of macroalgae have been negatively affecting the coasts of Brittany, France, for at least five decades, caused by excessive nitrogen inputs from the farming sector. Regular areal estimates of green tide surfaces are publicly available but only from 2002 onwards. Using free and openly accessible Landsat satellite imagery archives over 35 years (1984–2019), this study explores the potential of remote sensing for detection and long-term monitoring of green macroalgae blooms. By using a Google Earth Engine (GEE) script, we were able to detect and quantify green tide surfaces using the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) at four highly affected beaches in Northern Brittany. Mean green tide coverage was derived and analyzed from 1984 to 2019, at both monthly and annual scales. Our results show important interannual and seasonal fluctuations in estimated macroalgae cover. In terms of trends over time, green tide events did not show a decrease in extent at three out of four studied sites. The observed decrease in nitrogen concentrations for the rivers draining the study sites has not resulted in a reduction of green tide extents.

Original languageEnglish
Article number1408
JournalRemote Sensing
Volume13
Issue number8
DOIs
Publication statusPublished - 7 Apr 2021

Keywords

  • Coastal waters
  • Earth observation
  • Eutrophication
  • Google Earth Engine
  • Macroalgae
  • Remote sensing
  • Satellite

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